1. Field of the Invention
This invention relates to image-forming systems of the type wherein images are recorded by selectively applying a color developer onto a photosensitive/pressure sensitive recording medium which contains photocurable microcapsules.
2. Description of the Related Art
As is known in the art, color developers usually include ultrafine particles dispersed in aqueous solvents. A color developer sheet is fabricated by adding properly controlled binders and additives to the color developer, and then applying the mixture to a paper sheet. An image-forming technique using this type of color developer sheet is disclosed, for example, in U.S. Pat. Nos. 4,399,209, 4,483,912, 4,842,976 and others. The terminology image-forming technique as used herein is intended to mean all image forming techniques wherein two or more ingredients which have been initially separated from each other are brought into contact and combined with each other by application of a physical external force such as pressure and/or temperature, so that an optical variation, a variation in optical absorption region, or an absorption intensity is caused to occur, thereby rendering visible information present in the ingredients.
For example, one of the conventional image-forming techniques uses photocurable microcapsules as described below. This technique uses three types of photocurable microcapsules which, respectively, contain yellow, magenta and cyan dye precursors, dyes or pigments. The respective photocurable microcapsules have the following characteristics. The photocurable microcapsules which contain a cyan precursor, dye or pigment are cured by exposure to red light having a wavelength of approximately 650 nm. The photocurable microcapsules containing a magenta dye precursor, dye or pigment are cured by exposure to green light having a wavelength of approximately 550 nm. The photocurable microcapsules containing a yellow dye precursor, dye or pigment are cured by exposure to blue light having a wavelength of approximately 450 nm.
In order to record images utilizing the abovedescribed characteristics of the photocurable microcapsules, a photosensitive/pressure sensitive recording medium having these photocurable microcapsules applied thereon is used with a medium on which the image is to be recorded (a recording medium), and which has a color developer pre-coated thereon.
The photosensitive/pressure sensitive recording medium comprises a support (such as, for example, a web) having a photocurable microcapsule layer formed thereon. The support can have the photocurable microcapsule layer formed thereon by uniformly applying the three types of photocurable microcapsules onto the support. The recording medium (i.e., the medium which is to ultimately receive the image) also comprises a support having a color developer layer (which reacts with the dye-precursors or absorbs the pigments or dyes) uniformly coated on the support. If a dye precursor is contained in the respective photocurable microcapsules, the color developer provided on the recording medium is able to develop a color by reacting with the dye precursor (which is usually colorless). This reaction occurs when the recording medium is contacted with the photosensitive/pressure sensitive recording medium, and pressure or heat is applied to the combined mediums resulting in breakage of the microcapsules. Alternatively, when the photocurable microcapsules contain a dye or pigment (instead of the dye-precursor), the color is developed by absorption of the dyes or pigments by a dye-receiving material (on the recording medium) after breakage of the photocurable microcapsules as described above.
When red, green and blue light rays are irradiated, according to an original image, on the photocurable microcapsule layer of the photosensitive/pressure sensitive recording medium (which can be done simultaneously or at separate times for each different color), only the photocurable microcapsules which have been exposed to light having the predetermined wavelengths described above are cured. (This curing results in a change of the viscosity of the contents of the microcapsules, which in the described examples is an increase in viscosity, resulting in hardening of the microcapsules. This phenomenon is well known in the art.) This eventually leads to formation of a chemical latent image (corresponding to the original image) being formed on the photocurable microcapsules located on the support. Subsequently, the photosensitive/pressure sensitive recording medium and the recording medium are superposed such that the photocurable microcapsule layer on which the chemical latent image has been formed and the coated layer of color developer are in face-to-face contact. The superposed mediums are then passed between a pair of pressure rollers and/or heated to break uncured microcapsules by pressure and/or heat. The contents of the broken microcapsules are transferred to the recording medium, thereby developing and recording a desired image on the recording medium.
Another technique which is similar to the image-forming technique described above is described below. In the above-described image-forming technique, images are formed on a recording medium on which a developer has been pre-coated. The technique to be described below enables images to be formed on any arbitrary type of recording medium. In this second technique, color developer powder is electrostatically applied onto an arbitrary medium (such as, for example, plain paper), thereby forming a recording medium. Then, the same procedures (superposing, etc.) as in the first described technique are performed.
This second type of technique is known as a color developer application technique, and is disclosed in U.S. Pat. No. 5,038,710. According to the color developer application technique, an electrode is placed on the back side of any arbitrary recording medium. An electric field is generated between the electrode and a carrier (for example, a photosensitive drum) which is used to carry the color developer powder, so that the color developer powder is then electrostatically attracted to the recording medium. This results in the color developer powder being applied onto the entire surface of the arbitrary recording medium.
The application of the color developer particles onto the entire surface of the recording medium as described above has the following problem. When an image is formed on the recording medium which has the color developer powder applied on the entire surface thereof, the developer powder remains on the white or background regions even though no color developing function is required for these background regions. In the described example, the regions which are free of any image correspond to the area of the recording medium which was superposed with regions of the photosensitive/pressure sensitive recording medium that contained all cured microcapsules, and thus no color. Thus, the developer powder located on the background regions is wasted, increasing a cost per unit output image (hard copy). Moreover, the color developer powder located on the white (background) regions becomes discolored by prolonged exposure to light or air. This discoloration of the developer lowers the image contrast and degrades the storage properties of the resulting output image.
To avoid this problem, there has been proposed, for example, in U.S. Pat. No. 4,985,727, a selective application process of a color developer wherein color developer particles are selectively applied only onto portions of the photosensitive/pressure sensitive recording medium where color images are to be formed. In this process, however, the step of curing microcapsules by exposure of the photosensitive/pressure sensitive recording medium to a light image and the step of selectively applying a color developer onto the uncured portions of the light exposed photosensitive/pressure sensitive medium are performed separately. Accordingly, it is very difficult to register the selectively applied color developer and the uncured microcapsules. If the color developer and the latent microcapsule image are not accurately registered, not only a portion of the color developer is wasted by not contributing to the color development process, but also a portion of the desired image (i.e., the uncured microcapsules that were not coated with developer) are not color developed. Thus, there arises the problem that the image is incomplete and unclear.